1. Field of the Invention
The present invention relates in general to loudspeakers and in particular to a diaphragm for a loudspeaker that significantly improves the quality of sound and the usable life of the loudspeaker.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A typical loudspeaker transducer 10, as shown in FIG. 1, has a cone 12 and/or dome 14, diaphragm that is driven by a voice coil 16 that is immersed in a strong magnetic field. The voice coil 16 is electrically connected to an amplifier and, when in operation, the voice coil 16 moves back and forth in response to the electromagnetic forces on the coil caused by the current in the coil, generated by the amplifier, and the stationary magnetic field. The cone 12 and voice coil 16 assembly is typically suspended by a xe2x80x9cspiderxe2x80x9d 18 and a xe2x80x9csurroundxe2x80x9d 13, a flexible connector to frame 20. This suspension system allows the cone and coil assembly to move as a finite excursion piston over a limited frequency range. Like all mechanical structures, cones and domes have natural modes or xe2x80x9cMode peaksxe2x80x9d commonly called xe2x80x9ccone break-upxe2x80x9d. The frequency at which these modes occur is largely determined by the stiffness, density, and dimensions of the diaphragm, and the amplitude of these modes is largely determined by internal damping of the diaphragm material. These mode peaks are a significant source of audible coloration and, as a result, degrade the performance of the loudspeaker system.
Designers have tended to take two paths to solve the cone break-up problem. For small diaphragms such as those found in dome tweeters, aluminum and titanium are commonly used. In these applications, the dome dimensions can be manipulated such that the first natural modes of the dome are above the frequency range of human hearing. FIG. 2 shows the frequency response of a typical 1xe2x80x3 titanium dome tweeter (note the large mode peak 22 at 25 kHz). The amplitude of these modes is usually very high because metals have very little internal damping. For diaphragms larger than approximately 1xe2x80x3. the dome modes tall into the audible range. These modes are plainly audible as coloration because of the high amplitude of the modes. FIG. 3 shows the frequency response of a typical 3xe2x80x3 titanium dome mid-range speaker (note several large peaks 24, 26, and 28 at 11 kHz, 16 KHz, and 18 kHz).
For larger diaphragms, softer materials such as polymers or papers are commonly used. These materials have several natural modes in the band in which they operate. However, the internal damping of these materials is high enough so that most of these modes do not cause audible coloration. The remaining modes are either compensated for in other parts of the loudspeaker system design, resulting in increased costs, or are not addressed at all, resulting in lower performance. FIG. 4 shows the frequency response of a typical 5xe2x80x3 wooler with a polypropylene cone (note the large mode peaks 30 and 32 at 4 kHz and 5 kHz).
Many metal diaphragms feature a thin anodized layer. Typically, the metal is anodized to provide a specific color to the visible surface, or to protect the metal from sunlight, humidity, or moisture.
Ceramic materials such as alumina or magnesia offer significantly higher stiffness numbers and slightly better internal losses than typical metals such as titanium or aluminum. As a result, the natural modes of diaphragms made of these materials are moved higher in frequency and reduced in amplitude and, thus, reduce audible coloration. For instance, FIG. 14 shows the frequency response of a 5xe2x80x3 woofer with a ceramic metal matrix cone of the present invention. Note that the mode peaks 34 and 36 occur at approximately 6.5 kHz and 8.5 kHz. Compare FIG. 14 to FIG. 4. The mode peaks 34 and 36 have moved to a significantly higher frequency than mode peaks 30 and 32 in FIG. 4. This frequency extension allows a more simple and economical roll-off circuit, well known in the art, to be constructed to eliminate the unwanted frequencies.
Table I shows the important structural parameters for several materials. Unfortunately, pure ceramics are very brittle and are prone to shattering when used as loudspeaker diaphragms. Additionally, making diaphragms of appropriate dimensions can be very expensive. As a result, pure ceramic loudspeaker diaphragms have not become common.
Thus, the present invention relates to a speaker diaphragm material that is formed of a matrix, or layers, of a light metal such as aluminum, sandwiched between two ceramic layers, preferably aluminum oxide (Al2O3). The material is particularly useful as a loudspeaker diaphragm. The ceramics, Al2O3, are generally stiffer than metals and also offer improved damping. A loudspeaker diaphragm made of aluminum oxide would offer performance superior to any of the known materials today. Unfortunately, ceramics are also very brittle, and a diaphragm made of pure aluminum oxide would xe2x80x9cshatter itself to bitsxe2x80x9d under normal loudspeaker operations.
Thus, the material of the present invention is made of two layers of ceramic separated by a light metal substrate. Of the common metals, aluminum has the lowest density, making it the ideal substrate. However, there is no known reason why other metals, such as copper, titanium, and the like should not have the same advantages as the use of aluminum.
A skin of alumina, or ceramic, is formed by well-known means, such as anodizing and/or being xe2x80x9cgrownxe2x80x9d, on each side of the aluminum core or substrate. Anodizing provides a molecular bond instead of a chemical bond between the substrate and the ceramic material. The alumina thus supplies the strength and the aluminum substrate supplies the resistance to shattering. It has high internal frequency losses. The resulting composite material is less dense and less brittle than traditional ceramics, yet is significantly stiffer, and has better damping than titanium. It also resists moisture and sunlight better than any polymer and is at least as good as other metals for providing such resistance.
Thus, it is an object of the present invention to provide a loudspeaker diaphragm formed of composite material.
It is also an object of the present invention to provide a loudspeaker diaphragm formed of a composite material that is less dense and less brittle than traditional ceramics, yet it is significantly stiffer and has better damping than titanium.
It is a further object to the present invention to provide a loudspeaker diaphragm that resists moisture and sunlight to a greater degree than any polymer or most metal diaphragms.
It is still another object of the present invention to provide a loudspeaker diaphragm material formed of a composite source of two layers of ceramic material separated by a light metal substrate.
It is still another object to the present invention to provide a speaker diaphragm formed of a layer of light metal, or substrate, having an increased oxide layer on each side and wherein the preferred percentage ratio of ceramic layers to the light metal substrate core is 33⅓%, 33⅓%, and 33⅓%.
It is also an object of the present invention to provide a speaker diaphragm formed of a composite material such as two layers of ceramic material having a thickness of at least about 1 mil. and separated by a light metal substrate.
It is also an object of the present invention to provide a material wherein two layers of ceramic material are separated by a light metal substrate, such as aluminum, and wherein the ceramic layers are formed of Al2O3.